There are presently two known techniques in which nanovolt-meters may reduce amplifier offset drift. The first technique, generally referred to as the measure zero method and illustrated by FIG. 1, includes a two-phase measurement: a measuring of the input, and a measuring of the circuit common. The difference between the two measurements may be used to remove the offsets of the amplifier A and attenuator and of the measurement circuit, e.g., analog-to-digital converter (ADC).
When the switch 102 is in the first state, e.g., at Hi (as shown), the resulting voltage VOUT—1 is given by the following:
            V                        OUT          ⁢          _                ⁢        1              +          V              M        OS              =                    (                              +                          V              IN                                +                      V            OS                    +                      V                          T              ⁢                                                          ⁢              1                                      )            ⁢              (                                            R              1                        +                          R              2                                            R            1                          )              +          V              M        OS            
When the switch 102 is in the second state, e.g., at Lo (not shown), the resulting voltage VOUT—2 is given by the following:
            V                        OUT          ⁢          _                ⁢        2              +          V              M        OS              =                    (                              V            OS                    +                      V                          T              ⁢                                                          ⁢              1                                      )            ⁢              (                                            R              1                        +                          R              2                                            R            1                          )              +          V              M        OS            
Thus, the measurement voltage VMeas may be determined by the following:
      V    Meas    =            (                        V                                    OUT              ⁢              _                        ⁢            1                          +                  V                      M            OS                          -                  V                                    OUT              ⁢              _                        ⁢            2                          -                  V                      M            OS                              )        =                  V        IN            ⁡              (                                            R              1                        +                          R              2                                            R            1                          )            
The second technique, generally referred to as the chopping method and illustrated by FIG. 2, also includes a two-phase measurement but, in this technique, the differential inputs of the amplifier A and the amplifier's output are reversed, thereby producing two measurements that may be averaged to remove the amplifier's offset.
When the switch 202 is in the first state (as shown), the resulting voltage VOUT—1 is given by the following:
            V                        OUT          ⁢          _                ⁢        1              +          V      MOS        =                    (                              +                          V              IN                                +                      V            OS                    +                      V                          T              ⁢                                                          ⁢              1                                      )            ⁢              (                                            R              1                        +                          R              2                                            R            1                          )              +          V      MOS      
When the switch 202 is toggled (not shown), the resulting voltage VOUT—2 is given by the following:
            V                        OUT          ⁢          _                ⁢        2              +          V      MOS        =                    (                              +                          V              IN                                -                      V            OS                    +                      V                          T              ⁢                                                          ⁢              1                                      )            ⁢              (                                            R              1                        +                          R              2                                            R            1                          )              +          V      MOS      
Thus, the measurement voltage VMeas may be determined by the following:
      V    Meas    =                    (                              V                                          OUT                ⁢                _                            ⁢              1                                +                      V                                          OUT                ⁢                _                            ⁢              2                                +                      2            ⁢                                                  ⁢                          V              MOS                                      )            2        =                            (                                    V              IN                        +                          V                              T                ⁢                                                                  ⁢                1                                              )                ⁢                  (                                                    R                1                            +                              R                2                                                    R              1                                )                    +              V        MOS            
Because both measurements in this technique measure the input, the measurement noise is half compared to that of the gain stage illustrated by FIG. 1 but, because the attenuator offset is not removed, care must be taken to insure that the attenuator offset is stable. Also, the measurement circuit offset and noise, such as the ADC, are not removed and, thus, must be removed by other measurement methods.
Accordingly, a need remains for a more stable measurement than provided by present circuits and, more particularly, for rejection of both amplifier offset and attenuator offset while measuring the input signal in both phases.